This application is directed to a light guide in which a highly reflective multilayer optical film is used to obtain efficient, uniform emission of diffuse light.
The prior art has evolved a variety of light guides which are capable of distributing light along the light guide for controlled emission at one or more regions remote from the light source. U.S. Pat. No. 4,750,798 exemplifies one such prior art light guide.
Such prior art light guides have predominantly reflective interior surfaces. Accordingly, light rays entering one end of the guide are reflected by the guide""s inner walls as the rays proceed to the other end of the guide. In many lighting applications, the light guide is designed to xe2x80x9cleakxe2x80x9d light in a controlled manner, such that the amount of light emitted from the guide per unit length is acceptably uniform along the entire length of the guide, or along the entire length of each of the light guide""s light emitting regions.
U.S. Pat. No. 4,984,144 (Cobb, Jr. et al) discloses a prismatic refractive optical lighting film (hereafter xe2x80x9cprism light guide wall materialxe2x80x9d) available from 3M, Inc. as xe2x80x9coptical lighting filmxe2x80x9d under product nos. 2300 or 2301, which has been used to form a variety of prior art light guides. A common objective of such prior art guides is to distribute light from a point source over an area, to efficiently uniformly illuminate the area. Generally, the objective is to maximize the efficiency with which light is distributed along and emitted from the guide, while minimizing glare. Light guides formed of prism light guide wall material can emit light uniformly and diffusely over large areas with low glare relative to the level of illumination. Such guides are fabricated relatively easily and inexpensively, fulfilling a useful role.
In popular types of such prior art light guides the prism light guide wall material typically forms a tubular conduit. An extraction mechanism is provided inside the conduit. Light rays which enter the conduit from a light source are ordinarily internally reflected by the wall material and guided along the conduit. But, certain light rays which encounter the extraction mechanism are reflected in such a way that they are able to escape through the prism light guide wall material. However, light guides of this type are relatively inefficient in extracting diffuse light from the guide. This is because the light rays are typically reflected a number of times by a reflective cover outside the prism light guide wall material before they are emitted from the desired light emitting portion of the guide. In light guides constructed with currently available materials, each such reflection results in an absorption loss of about 5% to 10%. The net result is that about 25% of the available light is lost to absorption, instead of being emitted from the guide. In other words, the extraction efficiency of prior art light guides formed of prism light guide wall material is only about 75%.
The extraction efficiency of a light guide formed of prism light guide wall material can be improved by using an alternative technique to extract light from the guide. In particular, instead of providing an extraction mechanism inside the prism light guide wall material, a number of holes are provided in the wall material over the region from which light is to be extracted from the guide, or the wall material is otherwise modified in that region. With this design, light rays which escape from the light guide generally undergo fewer reflections before escaping, so the overall extraction efficiency is higher. Typically, only about 5% of the extracted light is lost to absorption, so the extraction efficiency is about 95%, which compares favourably to the 75% value in the previous example. A further advantage is that in some cases it is desirable that the escaping light be highly directional, as in the illumination of very high narrow architectural spaces. Such directionality is readily achieved with this alternative extraction technique.
Despite the advantages of the foregoing alternative extraction technique it has not become popular, for two reasons. First, light guides constructed in accordance with this technique are generally considered to be less visually attractive. For example, the light emitting surface is perceived as having a non-uniform distribution of light intensity, and as being too bright in some places. Second, it is much more difficult to design and manufacture light guides which employ the alternative extraction technique. Prototyping such designs typically involves an iterative procedure in which a substantial amount of valuable prism light guide wall material is unavoidably destroyed. Manufacturing such designs requires complex patterning technology to yield the optimal distribution of extraction effect over the surface of the prism light guide wall material.
The present invention overcomes the foregoing problems by exploiting the properties of newly discovered multilayer optical film materials.
In accordance with the preferred embodiment, the invention provides a light guide having a non-light emitting portion made up of multiple layers of first and second substantially non-absorptive longitudinally specular light reflector materials. The index of refraction of each layer differs from the indices of refraction of the immediately adjacent layers, such that the layers collectively have high longitudinally specular reflectivity. The light guide also has a light emitting portion comprising prism light guide wall material. A substantially non-absorptive light scattering mechanism is positioned within the non-light emitting portion in opposition to the light emitting portion.
The multiple-layered materials which make up the non-light emitting portion of the light guide may alternatively be such that each layer has a stress induced birefringence, with the layers collectively being birefringent over a range of uniaxial to biaxial orientation of the layers and having a collectively high longitudinally specular reflectivity. As another alternative, the layered materials may be first and second substantially non-absorptive polymers which differ in composition such that the layers collectively have high longitudinally specular reflectivity.
The light guide""s cross-section can be substantially constant, or its cross-section may decrease along the guide. In either case, the width of the light scattering mechanism may be increased as a function of length along the light guide.
Advantageously, a low absorption lens may be positioned outside the light guide""s light emitting portion. The light transmissivity characteristic of the lens may vary as a function of wavelength, polarization, angle, or a combination thereof.